Berkshire said in a regulatory filing late Tuesday that it bought 1.62 million more shares of the Ft. Worth-based railroad. It already was Burlington Northern's biggest shareholder, with 39 million shares, or 11 percent, as of April, according to data compiled by Bloomberg.

Berkshire paid $79.45 to $80.40 for the shares, which it bought in three separate transactions on Aug. 3, 6 and 7, according to the filing with the Securities and Exchange Commission.

Buffett said in May that he also bought shares of Union Pacific Corp. and Norfolk Southern Corp., the nation's largest and fourth-largest railroads, respectively.

He told shareholders at Berkshire's annual meeting in May that he likes railroad stocks because higher fuel prices have made the carriers more competitive with trucks. Berkshire spokeswoman Jackie Wilson had no comment Wednesday.